Electric linear motion actuator and electric brake system

ABSTRACT

An object is to allow smooth guidance of the linear motion of an output member even when lateral moment acts on the output member, and to make it possible to stably transmit torque of a rotary shaft to respective planetary rollers without applying a preload to the planetary rollers by negative gaps between the rollers and the radially outer surface of the rotary shaft and between the rollers and the radially inner surface of an outer race member. The axial movement of a carrier ( 6 ) supporting the planetary rollers ( 7 ) is restricted, and the outer race member ( 5 ) is axially slidably fitted in the radially inner surface of a cylindrical portion ( 1   a ) of a housing ( 1 ) and is rotationally fixedly coupled to an object to be driven by means of keys ( 25 ), whereby the outer race member ( 5 ) serves as a linearly driven output member. The planetary rollers ( 7 ) have their radially inner surfaces rotatably supported on respective support pins ( 6   c ) of the carrier ( 6 ), and are radially inwardly biased by radial compression ring springs ( 21 ) wound around the support pins ( 6   c ) so as to envelop the support pins. The planetary rollers ( 7 ) are thus pressed against the radially outer surface of the rotary shaft ( 4 ).

TECHNICAL FIELD

The present invention relates to an electric linear motion actuator forconverting the rotary motion of an electric motor to a linear motion ofa object to be driven, and an electric brake system including a memberto be braked and a braking member which is adapted to be pressed againstthe member to be braked by the electric linear motion actuator.

BACKGROUND ART

Many of electric linear motion actuators for converting the rotarymotion of an electric motor to a linear motion of an object to be driveninclude a ball-screw mechanism or a ball-ramp mechanism as a motionconverter means. Further, many of such linear motion actuators include agear reduction mechanism such as a planetary gear speed reducer so as toobtain a large linear driving force with a small-capacity electric motor(see e.g. Patent document 1).

The ball-screw mechanism or ball-ramp mechanism used in theabove-described electric linear motion actuators has the ability toincrease the driving force to some extent because it includes threadshaving a lead angle or inclined cam surfaces. But this motion convertingmechanism alone cannot sufficiently increase the driving force asrequired by e.g. an electric brake system. Thus, the electric linearmotion actuators using this type of motion converter mechanism furtherinclude a separate speed reducer such as a planetary gear speed reducerto increase the driving force. But the addition of such a separate speedreducer increases the size of the entire electric linear motionactuator.

The inventors of this application proposed an electric linear motionactuator which is free of this problem, which can sufficiently increasethe driving force without mounting a separate speed reducer, and whichcan be used in an electric brake system, of which the linear motionstroke is relatively short. This actuator comprises a rotary shaft towhich the rotation of a rotor shaft of an electric motor is configuredto be transmitted, a housing having a radially inner surface, an outerrace member fitted in the radially inner surface of the housing andprovided around the rotary shaft, a carrier, and a plurality ofplanetary rollers disposed between the outer race member and the rotaryshaft and rotatably supported by the carrier, wherein the planetaryrollers are configured to revolve around the rotary shaft while rotatingabout axes of the respective planetary rollers when the rotary shaftrotates, wherein a helical rib is formed on the radially outer surfaceof the rotary shaft or on the radially inner surface of the outer racemember, and wherein each of the planetary rollers has, on a radiallyouter surface thereof, a plurality of circumferential grooves which arearranged at the same pitch as the helical rib and in which the helicalrib is engaged, or a helical groove in which the helical rib is engaged,the helical groove having a different lead angle from the helical riband being arranged at the same pitch as the helical rib, thereby axiallymoving the carrier. Thus, the rotation of the rotary shaft is convertedto a linear motion of the carrier. In this arrangement, the carrier or alinear motion member coupled to the carrier serves as an output memberfor linearly driving an object to be driven (Patent documents 2 and 3).

On the other hand, while many of vehicle brake systems are hydraulicones, with the recent introduction of sophisticated brake control suchas anti-lock brake control system (ABS), electric brake systems aregathering attention because electric brake systems can perform suchsophisticated brake control without the need for complicated hydrauliccircuits. In an electric brake system, an electric motor is activated inresponse to a signal produced when the brake pedal is depressed, therebypressing the braking member against the member to be braked through theabove-described electric linear motion actuator, which is mounted in thecaliper body (see e.g. Patent document 4).

PRIOR ART DOCUMENTS Patent Documents

Patent document 1: JP Patent Publication 6-327190A

Patent document 2: JP Patent Publication 2007-32717A

Patent document 3: JP Patent Publication 2007-37305A

Patent document 4: JP Patent Publication 2003-343620A

SUMMARY OF THE INVENTION Object of the Invention

The electric linear motion actuator disclosed in Patent document 2 or 3,in which the carrier or the linear motion member coupled to the carrierserves as an output member which makes a linear motion, can sufficientlyincrease the driving force without the need to mount a separate speedreducer and thus is compact in size. But since the carrier or the linearmotion member, which makes a linear motion, is relatively short in theaxial direction, if this electric linear motion actuator is used e.g. inan electric brake system, a tangential force applied from the member tobe braked to the braking member as the object to be driven partiallyacts on the carrier or the linear motion member as lateral moment. Thislateral moment may hamper smooth linear motion of the carrier or thelinear motion member. If a linear motion member guided by the outer racemember is coupled to the brake member, as the braking member becomesworn, the axial length of the portion of the linear motion protrudingfrom the outer race member increases while the axial length of itsportion guided by the outer race member correspondingly decreases. Thismakes smooth linear motion of the linear motion member even moredifficult.

Also, in order to stably transmit of the rotary shaft as the input shaftto the respective planetary rollers, the planetary rollers are disposedbetween the rotary shaft and the outer race member with negative gaps,thereby applying a preload to the respective planetary rollers betweenthe radially outer surface of the rotary shaft and the radially innersurface of the outer race member. This makes it necessary to mount theplanetary rollers between the outer race member and the rotary shaft bye.g. interference fit. Also, in order to control the negative gaps forapplying the preload, it is necessary to finish the radially outersurface of the rotary shaft and the radially inner surface of the outerrace member with high accuracy by e.g. grinding. It is thus troublesomeand time-consuming to finish the rotary shaft and outer race member andthen mount the planetary rollers. This increases the manufacturing cost.

An object of the present invention is to allow smooth guidance of thelinear motion of the output member even when lateral moment acts on theoutput member, and to make it possible to stably transmit torque of therotary shaft to the respective planetary rollers without applying apreload to the planetary rollers by negative gaps between the rollersand the radially outer surface of the rotary shaft and between therollers and the radially inner surface of the outer race member.

In order to achieve this object, the present invention provides anelectric linear motion actuator comprising a rotary shaft to which therotation of a rotor shaft of an electric motor is configured to betransmitted, a housing having a radially inner surface, an outer racemember fitted in the radially inner surface of the housing and providedaround the rotary shaft, a carrier having a plurality of support pins,and a plurality of planetary rollers disposed between the outer racemember and the rotary shaft and rotatably supported by the respectivesupport pins of the carrier, wherein the planetary rollers areconfigured to revolve around the rotary shaft while rotating about axesof the respective planetary rollers when the rotary shaft rotates,wherein a helical rib is formed on a radially inner surface of the outerrace member, and wherein each of the planetary rollers has, on aradially outer surface thereof, a plurality of circumferential grooveswhich are arranged at the same pitch as the helical rib and in which thehelical rib is engaged, or a helical groove in which the helical rib isengaged, the helical groove having a different lead angle from thehelical rib and being arranged at the same pitch as the helical rib,thereby axially moving the outer race member and the carrier relative toeach other, whereby the rotary motion of the rotary shaft is convertedto a linear motion of an output member, thereby driving an object to bedriven coupled to the output member, wherein the axial movement of thecarrier is restricted, and the outer race member is rotationally fixedand is axially slidably fitted in the radially inner surface of thehousing, whereby the outer race member serves as the output member, andthat the actuator further comprises an elastic member biasing theplanetary rollers against a radially outer surface of the rotary shaft.

In this arrangement, since the carrier is axially immovable, and theouter race member is fitted in the housing so as to be non-rotatable butaxially movable so that the outer race member serves as the outputmember, it is possible to guide the outer race member as the outputmember over its axially long area with the radially inner surface of thehousing. This in turn makes it possible to smoothly guide the linearmotion of the output member even when lateral moment acts on the outputmember. By providing the elastic member biasing the planetary rollersagainst a radially outer surface of the rotary shaft, it is possible tostably transmit torque of the rotary shaft to the respective planetaryrollers without applying a preload to the planetary rollers by negativegaps between the rollers and the radially outer surface of the rotaryshaft and between the rollers and the radially inner surface of theouter race member.

As means for biasing the planetary rollers, the support pins may bemounted to the carrier so as to be circumferentially immovable andradially movable, and rotatably support the respective planetary rollersat a radially inner surface of each planetary roller, and the elasticmember biases the respective support pins radially inwardly toward therotary shaft.

The elastic member may be a ring-shaped elastic member enveloping thesupport pins so as to tend to be radially compressed.

The ring-shaped elastic member may be a radial compression spring madeof spring steel and having circumferentially spaced apart ends.

Preferably, the rotary shaft is supported by a single bearing at oneaxial position of the rotary shaft, and the rotary shaft is indirectlysupported by an annular member having an annular portion configured tobe brought into contact with the radially inner surface of the outerrace member when the annular portion minutely moves in a radialdirection at an axial position of the rotary shaft axially spaced fromthe axial position of the rotary shaft where the rotary shaft issupported by the single bearing. With this arrangement, even when theplanetary rollers are pressed against the radially outer surface of therotary shaft, no moment loads acts on the bearing for the rotary shaft.

The rotary shaft may be indirectly supported by the annular member at aplurality of axial positions of the rotary shaft which are axiallyspaced apart from each other and from the axial position of the rotaryshaft where the rotary shaft is supported by the single bearing so as tomore reliably prevent moment loads from acting on the bearing for therotary shaft.

Preferably, the annular member is brought into contact with the radiallyinner surface of the outer race member through a slide bearing so as toreduce friction loss when the rotary shaft rotates.

The slide bearing is preferably press-fitted on the annular member.

The present invention also provides an electric brake system comprisinga member to be braked, a braking member, and an electric linear motionactuator for converting a rotary motion of an electric motor to a linearmotion of the braking member, thereby pressing the braking memberagainst the member to be braked, wherein the electric linear motionactuator is of the above-described type. With this arrangement, evenwhen a tangential force applied from the braked member to the brakingmember acts on the output member as lateral moment, it is possible tosmoothly guide the linear motion of the output member.

ADVANTAGES OF THE INVENTION

According to the present invention, since the carrier is axiallyimmovable, the outer race member is fitted in the housing so as to benon-rotatable but axially movable so that the outer race member servesas the output member, and the elastic member is provided that pressesthe planetary rollers against the radially outer surface of the rotaryshaft, it is possible to smoothly guide the linear motion of the outputmember even when lateral moment acts on the output member, and to stablytransmit torque of the rotary shaft to the respective planetary rollerswithout applying a preload to the planetary rollers by negative gapsbetween the rollers and the radially outer surface of the rotary shaftand between the rollers and the radially inner surface of the outer racemember.

In the electric brake system according to the present invention, thebraking member to be pressed against the braked member is drivenlinearly using the above-mentioned linear motion actuator. Thus, evenwhen a tangential force applied from the braked member to the brakingmember acts on the output member as lateral moment, it is possible tosmoothly guide the linear motion of the output member.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view of an electric linear motionactuator embodying the present invention.

FIG. 2 is a sectional view taken along line II-II of FIG. 1.

FIG. 3 is a sectional view taken along line III-III of FIG. 1.

FIG. 4 is a side view of a ring-shaped radial compression spring shownin FIG. 1.

FIGS. 5( a) and 5(b) are front views showing a helical rib on the outerrace member of FIG. 1 and a helical groove on a planetary roller of FIG.1.

FIG. 6 is a vertical sectional view of an electric brake system usingthe electric linear motion actuator of FIG. 1.

BEST MODE FOR EMBODYING THE INVENTION

Now the embodiment of the invention is described with reference to thedrawings. As shown in FIGS. 1 to 3, the electric linear motion actuatorof the embodiment includes a housing 1 having a cylindrical portion 1 aand a flange 1 b extending to one side from one end of the cylindricalportion 1 a. An electric motor 2 is mounted to the flange lb so as to beparallel to the cylindrical portion 1 a. The electric motor 2 has arotor shaft 2 a of which the rotation is transmitted to a rotary shaft 3extending along the axis of the cylindrical portion 1 a through gears 3a, 3 b and 3 c. Four planetary rollers 7 are disposed between the rotaryshaft 4 and an outer race member 5 axially slidably fitted in theradially inner surface of the cylindrical portion 1 a. The planetaryrollers 7 are rotatably supported by a carrier 6, and adapted to revolvearound the rotary shaft 4 while rotating about their own axes when therotary shaft 4 rotates.

A lid 1 c is mounted to the housing 1 at its end where the flange 1 b isprovided. The gears 3 a, 3 b and 3 c mesh each other in the same planein the closed space defined by the lid 1 c. A shaft support member 8 ismounted in the cylindrical portion 1 a at its end portion where the lid1 c is mounted. The rotary shaft 4 has its proximal end portion, wherethe gear 3 c is mounted, supported by the shaft support member 8 througha ball bearing 9. The shaft support member 8 is fixed to the housing 1by means of snap rings 10 a and 10 b, and serves to restrict axialmovements of the rotary shaft 4 and the carrier 6 too. The intermediategear 3 b, which meshes with the gear 3 a, which is mounted to the rotorshaft 2 a, and the gear 3 c is supported by a shaft pin 11 extendingbetween the flange 1 b and the lid 1 c through a ball bearing 12.

The carrier 6 comprises a carrier body 6 a and a support plate 6 b whichare spaced from each other and rotatably fitted around the rotary shaft4 through slide bearings 13 a and 13 b, respectively, support pins 6 chaving their ends supported by the carrier body 6 a and the supportplate 6 b, respectively, and rotatably supporting the respectiveplanetary rollers 7, and a plurality of coupling rods 6 d through whichthe support plate 6 b and the carrier body 6 a are coupled together soas to be angularly aligned with each other. Each coupling rod 6 d hasits both ends coupled to the carrier body 6 a and the support plate 6 bby bolts 6 e, respectively.

The carrier body 6 a is supported on the end surface of the shaftsupport member S by means of a thrust roller bearing 14 through asupport member 6 f so as to be rotatable about the rotary shaft togetherwith the planetary rollers 7. The shaft support member 8 thus restrictsthe movement of the carrier body 6 a toward the proximal end of therotary shaft 4. A snap ring 15 fitted on the rotary shaft 4 at itsdistal end prevents separation of the support plate 6 b, which iscoupled to the carrier body 6 a through the coupling rods 6 d, through aslide bearing 16 and restricts its movement toward the distal end of therotary shaft 4. Thus, the movement of the carrier 6 toward either end ofthe rotary shaft 4 is restricted.

The planetary rollers 7 are rotatably supported on the respectivesupport pins 6 c of the carrier 6 through needle roller bearings 17 andare supported on the carrier body 6 a through thrust roller bearings 18so as to be rotatable about their own axes. Each support pin 6 c has itsends received in radially elongated holes 19 formed in the carrier body6 a and the support plate 6 b, respectively, so as to becircumferentially immovable and radially movable.

Grooves 20 are formed on the radially outer surface of each support pin6 c at the respective end portions thereof. Radial compression ringsprings 21 made of spring steel and circumferentially spaced apart endsas shown in FIG. 4 are fitted in the grooves 20 at the respective endsof the support pins 6 c in a radially expanded state so as to envelopthe support pins 6 c. The planetary rollers 7, which are rotatablysupported by the respective support pins 6 c, are thus pressed againstthe radially outer surface of the rotary shaft 4 by the springs 21. Thisallows stable transmission of torque of the rotary shaft 4 to therespective planetary rollers 7.

The coupling rods 6 d are disposed between the respective adjacentplanetary rollers 7 and couple the carrier body 6 a to the support plate6 b. A fan-shaped lubricant applicator member 22 is held in positionbetween each coupling rod 6 d and the radially inner surface of theouter race member 5 which is in sliding contact with the radially outersurfaces of the planetary rollers 7 on both circumferential sides toapply grease thereto. A seal member 23 is fitted on the radially innersurface of the outer race member 5 for isolating the interior of theouter race member, in which the planetary rollers 7 and the lubricantapplicator members 22 are mounted, from the outside. The seal member 23is made by pressing a thin steel sheet and includes a cylindrical outerportion fitted in the outer race member 5.

The carrier body 6 a and the support plate 6 b are annular membersadapted to be brought into contact with the radially inner surface 4 ofthe outer race member 5 through slide bearings 24 a and 24 b whenminutely moved in a radial direction. The carrier body 6 a and thesupport plate 6 b thus indirectly support the rotary shaft 4, therebypreventing moment loads from acting on the ball bearing 9, whichsupports the rotary shaft 4. The slide bearings 24 a and 24 b arepress-fitted and fixed on the radially outer surfaces of the carrierbody 6 a and the support plate 6 b, respectively.

An object to be driven is coupled to the distal end of the outer racemember 5. The object to be driven is rotationally fixed to the outerrace member by means of keys 25 formed on the distal end surface of theouter race member. Thus, the outer race member 5, which is axiallyslidably fitted in the radially inner surface of the cylindrical portionla of the housing 1, is axially movable relative to the carrier 6, ofwhich the movements in both axial directions are restricted, and thusserves as an output member. The end of the cylindrical portion 1 a towhich the object to be driven is coupled is open. Thus the outer racemember 5 can make a linear motion with a long stroke because it canprotrude from the cylindrical portion 1 a. Even when the outer racemember 5 protrudes from the cylindrical portion, it is guided by theradially inner surface of the cylindrical portion 1 a over an axiallylong area, so that the outer race member can smoothly make a linearmotion.

As shown in FIG. 5( a), two helical grooves 5 a are formed on theradially inner surface of the outer race member 5, with which theplanetary rollers 7 are brought into rolling contact. Separate ribmembers 5 b are fixedly fitted in the respective helical grooves 5 a,forming two helical ribs on the radially inner surface of the outer racemember 5. As shown in FIG. 5( b), a single helical groove 7 a is formedon the radially outer surface of each planetary roller 7, which is thesame in pitch as and different in lead angle from the helical ribsformed by the rib members 5 b, and in which the helical ribs areengaged. Due to the difference in lead angle between the helical ribsand the helical grooves 7 a, when the rotary shaft 4 rotates and theplanetary rollers 7 revolve around the rotary shaft 4 while rotatingabout their own axes, with the helical ribs engaged in the helicalgrooves 7 a, the outer race member 5 moves axially relative to theplanetary rollers 7. In the embodiment, two helical ribs are formed onthe outer race member so that the difference in lead angle between thehelical ribs and the helical grooves 7 a of the planetary rollers 7 canbe determined more freely. But a single helical rib may be formedinstead. Instead of the helical groove 7 a, a plurality ofcircumferential grooves may be formed on each planetary roller 7 a atthe same pitch as the helical ribs.

FIG. 6 shows an electric brake system in which the above-describedelectric linear motion actuator is used. This electric brake system is adisk brake including a caliper body 31, a disk rotor 32 as a member tobe braked, and brake pads 33 as braking members provided on both sidesof the disk rotor 32 in the caliper body 31 so as to face each other.The housing 1 of the electric linear motion actuator is fixed to thecaliper body 31, and its outer race member 5 as the output member isrotationally fixed to one of the brake pads 33 as the object to bebraked through the keys 24 so that the brake pads 33 can be pressedagainst the disk rotor 32. In FIG. 6, the electric linear motionactuator is shown in section taken along a plane perpendicular to theplane containing the section of FIG. 1.

In the above embodiment, the helical ribs on the radially inner surfaceof the outer race member are formed by separate rib members fitted inthe helical grooves. But instead, the helical ribs may be formedintegral with the outer race member.

DESCRIPTION OF THE NUMERALS

-   1. Housing-   1 a. Cylindrical portion-   1 b. Flange-   1 c. Lid-   2. Electric motor-   2 a. Rotor shaft-   3 a, 3 b, 3 c. Gear-   4. Rotary shaft-   5. Outer race member-   5 a. Helical groove-   5 b. Rib member-   6. Carrier-   6 a. Carrier body-   6 b. Support plate-   6 c. Support pin-   6 d. Coupling rod-   6 e. Bolt-   6 f. Support member-   7. Planetary roller-   7 a. Helical groove-   8. Shaft support member-   9. Ball bearing-   10 a, 10 b. Snap ring-   11. Shaft pin-   12. Ball bearing-   13 a, 13 b. Slide bearing-   14. Thrust roller bearing-   15. Snap ring-   16. Slide bearing-   17. Needle roller bearing-   18. Thrust roller bearing-   19. Elongated hole-   20. Groove-   21. Radial compression ring spring-   22. Lubricant applicator member-   23. Seal member-   24 a, 24 b. Slide bearing-   25. Key-   31. Caliper body-   32. Disk rotor-   33. Brake pad

1. An electric linear motion actuator comprising a rotary shaft to whichthe rotation of a rotor shaft of an electric motor is configured to betransmitted, a housing having a radially inner surface, an outer racemember fitted in the radially inner surface of the housing and providedaround the rotary shaft, a carrier having a plurality of support pins,and a plurality of planetary rollers disposed between the outer racemember and the rotary shaft and rotatably supported by the respectivesupport pins of the carrier, wherein the planetary rollers areconfigured to revolve around the rotary shaft while rotating about axesof the respective planetary rollers when the rotary shaft rotates,wherein a helical rib is formed on a radially inner surface of the outerrace member, and wherein each of the planetary rollers has, on aradially outer surface thereof, a plurality of circumferential grooveswhich are arranged at the same pitch as the helical rib and in which thehelical rib is engaged, or a helical groove in which the helical rib isengaged, said helical groove having a different lead angle from thehelical rib and being arranged at the same pitch as the helical rib,thereby axially moving the outer race member and the carrier relative toeach other, whereby the rotary motion of the rotary shaft is convertedto a linear motion of an output member, thereby driving an object to bedriven coupled to the output member, characterized in that the axialmovement of the carrier is restricted, and the outer race member isrotationally fixed and is axially slidably fitted in the radially innersurface of the housing, whereby the outer race member serves as theoutput member, and that the actuator further comprises an elastic memberbiasing the planetary rollers against a radially outer surface of therotary shaft.
 2. The electric linear motion actuator of claim 1, whereinthe support pins are mounted to the carrier so as to becircumferentially immovable and radially movable, and rotatably supportthe respective planetary rollers at a radially inner surface of eachplanetary roller, and wherein said elastic member biases the respectivesupport pins radially inwardly toward the rotary shaft.
 3. The electriclinear motion actuator of claim 2, wherein the elastic member is aring-shaped elastic member enveloping the support pins so as to tend tobe radially compressed.
 4. The electric linear motion actuator of claim3, wherein the ring-shaped elastic member is a radial compression springmade of spring steel and having circumferentially spaced apart ends. 5.The electric linear motion actuator of claim 1, wherein the rotary shaftis supported by a single bearing at one axial position of the rotaryshaft, and wherein the rotary shaft is indirectly supported by anannular member having an annular portion configured to be brought intocontact with the radially inner surface of the outer race member whenthe annular portion minutely moves in a radial direction at an axialposition of the rotary shaft axially spaced from the axial position ofthe rotary shaft where the rotary shaft is supported by the singlebearing.
 6. The electric linear motion actuator of claim 5, wherein therotary shaft is indirectly supported by the annular member at aplurality of axial positions of the rotary shaft which are axiallyspaced apart from each other and from the axial position of the rotaryshaft where the rotary shaft is supported by the single bearing.
 7. Theelectric linear motion actuator of claim 5, wherein the annular memberis brought into contact with the radially inner surface of the outerrace member through a slide bearing.
 8. The electric linear motionactuator of claim 7 wherein the slide bearing is press-fitted on theannular member.
 9. An electric brake system comprising a member to bebraked, a braking member, and an electric linear motion actuator forconverting a rotary motion of an electric motor to a linear motion ofthe braking member, thereby pressing the braking member against themember to be braked, characterized in that the electric linear motionactuator is the electric linear motion actuator of claim
 1. 10. Theelectric linear motion actuator of claim 2, wherein the rotary shaft issupported by a single bearing at one axial position of the rotary shaft,and wherein the rotary shaft is indirectly supported by an annularmember having an annular portion configured to be brought into contactwith the radially inner surface of the outer race member when theannular portion minutely moves in a radial direction at an axialposition of the rotary shaft axially spaced from the axial position ofthe rotary shaft where the rotary shaft is supported by the singlebearing.
 11. The electric linear motion actuator of claim 3, wherein therotary shaft is supported by a single bearing at one axial position ofthe rotary shaft, and wherein the rotary shaft is indirectly supportedby an annular member having an annular portion configured to be broughtinto contact with the radially inner surface of the outer race memberwhen the annular portion minutely moves in a radial direction at anaxial position of the rotary shaft axially spaced from the axialposition of the rotary shaft where the rotary shaft is supported by thesingle bearing.
 12. The electric linear motion actuator of claim 4,wherein the rotary shaft is supported by a single bearing at one axialposition of the rotary shaft, and wherein the rotary shaft is indirectlysupported by an annular member having an annular portion configured tobe brought into contact with the radially inner surface of the outerrace member when the annular portion minutely moves in a radialdirection at an axial position of the rotary shaft axially spaced fromthe axial position of the rotary shaft where the rotary shaft issupported by the single bearing.
 13. The electric linear motion actuatorof claim 6, wherein the annular member is brought into contact with theradially inner surface of the outer race member through a slide bearing.14. An electric brake system comprising a member to be braked, a brakingmember, and an electric linear motion actuator for converting a rotarymotion of an electric motor to a linear motion of the braking member,thereby pressing the braking member against the member to be braked,characterized in that the electric linear motion actuator is theelectric linear motion actuator of claim
 2. 15. An electric brake systemcomprising a member to be braked, a braking member, and an electriclinear motion actuator for converting a rotary motion of an electricmotor to a linear motion of the braking member, thereby pressing thebraking member against the member to be braked, characterized in thatthe electric linear motion actuator is the electric linear motionactuator of claim
 3. 16. An electric brake system comprising a member tobe braked, a braking member, and an electric linear motion actuator forconverting a rotary motion of an electric motor to a linear motion ofthe braking member, thereby pressing the braking member against themember to be braked, characterized in that the electric linear motionactuator is the electric linear motion actuator of claim
 4. 17. Anelectric brake system comprising a member to be braked, a brakingmember, and an electric linear motion actuator for converting a rotarymotion of an electric motor to a linear motion of the braking member,thereby pressing the braking member against the member to be braked,characterized in that the electric linear motion actuator is theelectric linear motion actuator of claim
 5. 18. An electric brake systemcomprising a member to be braked, a braking member, and an electriclinear motion actuator for converting a rotary motion of an electricmotor to a linear motion of the braking member, thereby pressing thebraking member against the member to be braked, characterized in thatthe electric linear motion actuator is the electric linear motionactuator of claim
 6. 19. An electric brake system comprising a member tobe braked, a braking member, and an electric linear motion actuator forconverting a rotary motion of an electric motor to a linear motion ofthe braking member, thereby pressing the braking member against themember to be braked, characterized in that the electric linear motionactuator is the electric linear motion actuator of claim
 7. 20. Anelectric brake system comprising a member to be braked, a brakingmember, and an electric linear motion actuator for converting a rotarymotion of an electric motor to a linear motion of the braking member,thereby pressing the braking member against the member to be braked,characterized in that the electric linear motion actuator is theelectric linear motion actuator of claim 8.